1. Field of the Invention
The present invention relates to an optical recording medium including an information layer formed on a substrate, a spacer layer formed on the information layer, and another information layer formed on the spacer layer. The invention also relates to a method for manufacturing the same.
2. Description of the Related Art
Recently, optical recording media such as a CD (Compact Disc) and a DVD (Digital Versatile Disc) are rapidly spreading as information recording media. The optical recording medium is generally standardized to have an outer diameter of 120 mm and a thickness of 1.2 mm. In the case of the DVD, a laser beam having a shorter wavelength than that for the CD is used as irradiation light. In addition, a numerical aperture of a lens of the DVD for the irradiation light is set larger than that for the CD. As a result, the DVD is capable of recording and reproducing a larger amount of information at a higher density than the CD.
On the other hand, information recording and reproduction accuracy is more likely to lower as the wavelength of irradiation light becomes shorter and the numerical aperture of a lens becomes larger. This is because coma aberration occurs due to inclination and warp of a disc. Thus, the DVD includes a light transmitting layer having a half thickness of that of the CD, that is, 0.6 mm so as to ensure a margin for the inclination and the warp of the disc to maintain the information recording/reproduction accuracy.
Since the light transmitting layer at a thickness of 0.6 mm alone does not offer sufficient stiffness and strength, the DVD has such a structure that a pair of substrates, each having a thickness of 0.6 mm, are bonded to each other so that the information recording faces inside. As a result, the DVD has a thickness of 1.2 mm, which is equal to that of the CD, to ensure almost the same stiffness and strength as those of the CD.
In order to realize the recording of a larger amount of information at a higher density, the wavelength of irradiation light is further reduced while the numerical aperture of a lens is further increased. In response thereto, an optical recording medium including a light transmitting layer at a further reduced thickness has attracted attention (for example, see Japanese Patent Laid-Open Publication No. 2003-85836).
In order to standardize the specifications, a blue-violet laser beam having a wavelength of approximately 405 nm is used as irradiation light while the numerical aperture is set to 0.85. In correspondence with the laser beam and the numerical aperture, an optical recording medium including a light transmitting layer at a thickness of approximately 100 μm is now increasingly in widespread use. It is proposed that a track pitch of a convexo-concave pattern of pits or grooves (pitch of concave and convex portions in radial direction) of an information layer is set to approximately 320 nm.
The optical recording medium can also be formed as a dual-layer recording medium by forming a spacer layer on an information layer formed on either one of or both the surfaces of a substrate having a convexo-concave pattern such as a pit and a groove and then forming another information layer on the spacer layer. In order to standardize the specifications for the dual-layer recording medium, it is suggested to set a thickness of the spacer layer to approximately 25 μm and a thickness of the light transmitting layer to approximately 75 μm (total thickness of approximately 100 μm). Incidentally, the dual-layer recording medium may have two or more spacer layers to form three or more information layers.
As a technique of forming a plurality of information layers, each being in a convexo-concave patterned shape composed of pits and grooves, the following technique is known (for example, see Japanese Patent Laid-Open Publication No. 2003-91887). According to the technique, a convexo-concave pattern is first formed on a substrate by injection molding. An information layer is formed by sputtering or the like in accordance with the convexo-concave pattern. Next, an energy beam curable resin in a flowing state is applied onto the information layer and a light transmitting stamper is abutted on the energy beam curable resin to transfer the convexo-concave pattern thereto. After an energy beam such as an ultraviolet ray or an electron beam is radiated onto the energy beam curable resin through the light transmitting stamper to cure the energy beam curable resin, the light transmitting stamper is removed to form a spacer layer. Another information layer is formed by sputtering or the like in accordance with a convexo-concave pattern of the spacer layer.
If an information layer in a convexo-concave patterned shape having a track pitch (a radial pitch between convex and concave portions) of 320 nm is formed by the above-mentioned technique, however, electric characteristics such as a jitter or a noise of the information layer formed on the spacer layer are likely to be inferior to those of the information layer formed on the substrate. As a result, there is a problem that desired electric characteristics cannot be obtained for the information layer formed on the spacer layer.